What do you think are the advantages and disadvantages of each of them?
If you use ropes and constant force springs, the ropes can spool inconsistently which will be difficult to deal with from a code standpoint. I’m sure there are, however, ways that it can be done right.
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Constant force springs provide much less power for extending the arm. Unless you are lifting something light like a climber hook, they do not provide enough force to pull an elevator up quickly.
The ropes will only spool inconsistently if the spool an rigging is poorly designed. Properly tensioned ropes, small fleet angles, and grooved spools will behave very consistently.
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Good morning!
I’m an engineer at Vulcan Spring, a supporter of FIRST. We manufacture constant force springs and supply them to FIRST teams.
Constant force springs have various benefits: no reliance on battery performance (excluding release mechanisms), small size, and repeatable/consistent motion.
Regarding the point above (conforce springs’ use with cables), this can cause issues when the extended spring is able to twist. This is more likely when the extended spring is poorly supported or when there is significant friction in the system. Including a backer to the extended spring (or running it in a track) is great when possible.
I’m not very familiar with conventions for FIRST robot design, but I’m happy to answer any technical questions about conforce springs!
Have a good day!
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Great to see you here, I know we’ve used plenty of your springs in the past, including some on last year’s robot 
I think the question in this thread really comes down to how CF springs perform with dynamic loading, when compared to a motor-powered solution. With this game and the solution concept being explored in this thread, the spring would be used to power one direction of linear motion on a rotating arm, meaning the force on the mechanism the spring is pulling on due to gravity will be changing, as will the weight of the mechanism itself as it picks up and puts down game pieces.
I think it could be implemented to work, but a motor powered solution in this particular case might be a little more reliable given the dynamic situation.
Depends on the motor and its gearing, and the CF spring chosen. These springs can be quite strong if needed - but in most FRC use cases, we use them as counterbalance or for extension where a motor needs to power retraction, so the motor needs to be more powerful.
Thanks for the insights, Jon! I have just two ideas to offer:
- A varying load shouldn’t be a concern for the spring in and of itself, as long as the loading remains below the spring force. It’s worth considering inertia here also (over and above the nominal weights involved). To your point, a motor-driven mechanism could yield more consistent motion depending on the application.
- This may already be standard practice in FIRST, but it’s worth noting that conforce springs can be interwound to multiply force. In interwinding, multiple identical springs are layered together and mounted on the same drum. A quantity of (3) springs with 5 lbf force each will exert ~15 lbf when combined in this way. (Be careful with this operation! Ensure you’re protected against cuts and flying springs!) Interwinding can be finicky, so be sure to test well!
Thanks again for the insights! Best of luck with the competition!
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It seems so obvious when stated, yet something I know I had never considered or seen before - that is some great insight!
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My pleasure!
Thanks for sharing.
We buy from you folks all the time.
We just so happen were planning to buy a lot today. 
That’s great to hear! I hope they exceed your expectations!